An S parameter is one of circuit network parameters that are used to represent the characteristics of a high-frequency electronic circuit or a high-frequency electronic part, and is also called a scattering parameter. An electronic circuit system (even one high-frequency electronic part is considered as one small system) is assumed, which has n ports (in a system characterized by S parameters, a terminal is typically called a port) and which is characterized by S parameters. When a sine wave is input to one of the ports, the S parameters of the system represent a ratio at which the sine wave is transmitted to another one of the ports, a ratio at which the sine wave is reflected to the port to which the sine wave has been input, and phase delays of the transmitted wave and the reflected wave with respect to the input sine wave.
Each of the S parameters is a complex number. The absolute value of the complex number represents a transmission ratio (a reflection ratio), and an argument of the complex number represents a phase delay. The transmission ratio and the phase delay of a transmitted wave are collectively called a transmission coefficient, and the reflection ratio and the phase delay of a reflected wave are collectively called a reflection coefficient.
When multiple ports are present in a system, a sine wave is input to each of the ports, and a transmission coefficient and a reflection coefficient in this case are obtained. In other words, one reflection coefficient and n−1 transmission coefficients, that is, n parameters in sum, are obtained for one input port. Because the n parameters are present for each of the n input ports, n×n parameters are present. A file in which the S parameters are written is an S-parameter file.
When the frequency of the input sine wave changes, the characteristics of the system change. Accordingly, in the S-parameter file, the n×n parameters are written in a matrix for each frequency of the input sine wave.
In the case of printed wiring board design, a circuit simulator is used in transmission-line analysis for signal integrity (SI) verification or a power-supply board analysis for power integrity (PI) verification. As models to be analyzed, passive elements, such as resistors R, inductors L, and capacitors C, are used, and, in addition, elements (hereinafter, S elements) that represent, using S parameters, the characteristics of filters, vias, connectors, and transmission-line systems are also used.
The circuit simulator recognizes a circuit network in which the passive elements, the S elements, active elements such as diodes and complementary metal oxide semiconductor (MOS) transistors, and various power sources are electrically connected. An element is connected to a “node” that is electrically equipotential in the circuit network. Then, the circuit simulator calculates a change, over time, in a voltage at each node or a current flowing into each element in accordance with Kirchhoff's circuit laws that are basic laws of a circuit network theory.
Typically, in order to calculate the value of a current flowing into an S element, the circuit simulator performs inverse Fourier transform on S parameters to obtain an impulse response function, and calculates the convolution of the voltage of a terminal of the S element and the impulse response function in the time domain. Moreover, typically, the inverse fast Fourier transform (IFFT) algorithm is used as an algorithm of inverse Fourier transform using a computer. N pieces of data to be passed on to an IFFT function are extracted, from the S parameters, at frequencies that are set at regular intervals. As a condition for the IFFT algorithm, there is a condition where N is a power of 2.
As a method for obtaining S parameters, for example, a method in which actual measurement using a spectrum analyzer is performed, a method in which S parameters are calculated using an S-parameter extraction simulator, or a method in which an S-parameter library that is supplied from a circuit-part vendor is acquired is considered. Regarding the S parameters that have been acquired using any one of the methods, frequencies that are written are not necessarily frequencies provided at regular intervals. The number of frequencies (also referred to as the number of data items or the number of sampling points) is not necessarily a power of 2.
In a circuit simulator that is capable of supporting an S element, typically, various control parameters related to sampling for IFFT are prepared. More specifically, the control parameters are a sampling frequency interval (referred to as “FBASE”) of S parameters, a maximum frequency (FMAX) which indicates that sampling of the S parameters at frequencies up to the maximum frequency FMAX is performed, an interpolation method (INTERPOLATION) and extrapolation methods (LOWPASS and HIGHPASS) in the case of performing data interpolation, and so forth.
When the value of FBASE, FMAX, INTERPOLATION, LOWPASS, or HIGHPASS is changed, a result of calculation using IFFT (that is, an impulse response function) changes. Thus, the result of the convolution of the impulse response function and a signal changes, and, consequently, the result of circuit simulation changes. However, such control parameters are not appropriately set in the related art.
Japanese Laid-open Patent Publications No. 2000-346891, No. 11-295365, No. 08-262080, and No. 04-168907 are examples of the related art.